Mupirocin is bacteriostatic at low concentrations and bactericidal at high concentrations.[2] It is
used topically and is effective against Gram-positivebacteria, including MRSA.[3]
Mupirocin is a mixture of several pseudomonic acids, with
pseudomonic acid A (PA-A) constituting greater than 90% of the
mixture. Also present in mupirocin are pseudomonic acid B with an
additional hydroxyl group at C8,[4]
pseudomonic acid C with a double bond between C10 and C11, instead
of the epoxide of PA-A,[5] and pseudomonic acid D with
a double bond at C4` and C5` in the 9-hydroxy-nonanoic acid portion
of mupirocin.[6]

Uses

Mupirocin is used as a topical treatment for bacterial skin
infections, for example, furuncle, impetigo, open wounds etc. It is also useful
in the treatment of methicillin-resistant
Staphylococcus aureus (MRSA), which is a significant
cause of death in hospitalized patients who have received systemic
antibiotic therapy. It is suggested, however, that mupirocin cannot
be used for extended periods of time, or indiscriminately, as
resistance does develop, and could, if it becomes widespread,
destroy mupirocin's value as a treatment for MRSA. It may also
result in overgrowth of non-susceptible organisms.

Resistance

Shortly after the clinical use of Mupirocin began, strains of
Staphylococcus aureus that were resistant to mupirocin emerged.[10]
Two distinct populations of mupirocin-resistant S. aureus
were isolated. One strain possessed low-level resistance, MuL, (MIC
= 8-256 mg/L) and another possessed high-level resistance,
MuH, (MIC > 256 mg/L).[10]
Resistance in the MuL strains is probably due to mutations in the organism’s
wild-type isoleucinyl-tRNA synthetase. In E. coli IleRS, a
single amino acid mutation was shown to alter mupirocin
resistance.[11]
MuH is linked to the acquisition of a separate Ile synthetase gene,
mupA.[12]
Mupirocin is not a viable antibiotic against MuH strains. Other
antibiotic agents such as azelaic acid, nitrofurazone, silver
sulfadiazine, and ramoplanin have been shown to be effective
against MuH strains.[10]

The mechanism of mupirocin differs from other clinical
antibiotics rendering cross-resistance to other antibiotics
unlikely.[10]
However, the MupA gene may co-transfer with other antibacterial
resistance genes. This has been observed already with resistance
genes for triclosan, tetracycline, and trimethoprim.[10]

Biosynthesis

Advertisements

Biosynthesis of Pseudomonic
Acid A

The 74 kb mupirocin gene cluster contains six multi-domainenzymes and
twenty six other peptides (Table 1).[13]
Four large multi-domain type I polyketide synthase (PKS) proteins
are encoded, as well as several single function enzymes with
sequence similarity to type II PKSs.[13]
It is therefore believed that mupirocin is constructed by a mixed
type I and type II PKS system. The mupirocin cluster exhibits an
atypical acyltransferase (AT) organization, in
that there are only two AT domains and both are found on the same
protein, MmpC. These AT domains are the only domains present on
MmpC, while the other three type I PKS proteins contain no AT
domains.[13]
The mupirocin pathway also contains several tandem acyl
carrier protein doublets or triplets. This may be an adaptation
to increase the throughput rate or to bind multiple substrates
simultaneously.[13]

Table 1. The biosynthetic gene cluster of mupirocin

Pseudomonic acid A is the product of an esterification
between the 17C polyketide monic acid and the 9C fatty acid
9-hydroxy-nonanoic acid. The possibility that the entire molecule
is assembled as a single polyketide with a Baeyer-Villiger oxidation inserting an oxygen into the carbon backbone has been ruled
out because C1 of monic acid and C9’ of 9-hydroxy-nonanoic acid are
both derived from C1 of acetate.[14]

Monic
acid biosynthesis

Biosynthesis of the 17C monic acid unit begins on MmpD (Figure
2).[13]
One of the AT domains from MmpC may transfer an activated acetyl
group from acetyl-Coenzyme A (CoA) to the first ACP domain. The
chain is extended by malonyl-CoA, followed by a SAM-dependent
methylation at C12 (see Figure 1 for PA-A numbering) and reduction
of the B-keto group to an alcohol. The dehydration (DH) domain in
module 1 is predicted to be non-functional due to a mutation in the
conserved active site region. Module 2 adds another two carbons by
malonyl-CoA extender unit, followed by ketoreduction (KR) and
dehydration. Module three adds a malonyl-CoA extender unit,
followed by SAM-dependent methylation at C8, ketoreduction, and
dehydration. Module 4 extends the molecule with a malonyl-CoA unit
followed by ketoreduction.

Assembly of monic acid is continued by the transfer of the 12C
product of MmpD to MmpA.[13]
Two more rounds of extension with malonyl-CoA units are achieved by
module 5 and 6. Module 5 also contains a KR domain.

Figure 2. The domain structure of MmpA, MmpC, and MmpD for the
synthesis of monic acid. The biosynthesis of monic acid is not
colinear but has been rearranged in this diagram. The protein name
is displayed inside of the arrow with module and domain structure
listed below. ACP=acyl carrier protein, AT=acyl transferase,
DH=dehydratase, ER=enoyl reductase, HMG=3-hydroxy-3-methylglutaric
acid, MeT=methyl transferase, KR=ketoreductase, KS=ketosynthase,
TE=thioesterase.

Post-PKS
tailoring

The keto group at C3 is replaced with a methyl group in a
multi-step reaction (Figure 3). MupG begins by decarboxylating a
malonyl-ACP. The alpha carbon of the resulting acetyl-ACP is linked
to C3 of the polyketide chain by MupH. This intermediate is
dehydrated and decarboxylated by MupJ and MupK, respectively.[13]

Figure 3. The C15 methyl group of monic acid is attached to C3
by the following reaction scheme. MupH is an
3-hydroxy-3-methylglutarate-Coenzyme A synthase, MupJ and MupK are
Enoyl-CoA hydratases.[13]

The formation of the pyran
ring requires many enzyme mediated steps (Figure 4). The double
bond between C8 and C9 is proposed to migrate to between C8 and
C16.[15]Gene knockout
experiments of mupO, mupU, mupV, and macpE have eliminated PA-A
production.[15]
PA-B production is not removed by these knockouts, demonstrating
that PA-B is not created by hydroxylating PA-A. A knockout of mupW
eliminated the pyran ring, identifying MupW as being involved in
ring formation.[15]
It is not known if this occurs before or after the esterification
of monic acid to 9-hydroxy-nonanoic acid.

Figure 4. The pyran ring of mupirocin is generated in this
proposed multistep reaction [15]. Gene knockouts of mupO, mupU,
mupV and macpE abolish PA-A production but not PA-B production,
demonstrating that PA-B is a precursor to PA-A.[15]

The epoxide of PA-A at
C10-11 is believed to be inserted after pyran formation by a cytochrome P450
such as MupO.[13]
A gene knockout of mupO abolished PA-A production but PA-B, which
also conatins the C10-C11 epoxide, remained.[15]
This indicates that MupO is either not involved or is not essential
for this epoxidation step.

9-Hydroxy-nonanoic acid
biosynthesis

The nine carbon fatty acid 9-hydroxy-nonanoic acid (9-HN) is
derived as a separate compound and later esterified to monic acid
to form pseudomonic acid. 13C labeled acetate feeding has shown that C1-C6 are
constructed with acetate in the canonical fashion of fatty
acid synthesis. C7’ shows only C1 labeling of acetate while C8’
and C9’ show a reversed pattern of 13C labeled acetate.[14]
It is speculated that C7-C9 arises from a 3-hydroxypropionate
starter unit, which is extended three times with malonyl-CoA and
fully reduced to yield 9-HN. It has also been suggested that 9-HN
is initiated by 3-hydroxy-3-methylglutaric acid (HMG). This latter
theory was not supported by feeding of [3-14C] or
[3,6-13C2]-HMG.[6]

MmpB is proposed to catalyze the synthesis of 9-HN (Figure 5).
MmpB contains a KS, KR, DH, 3 ACPs, and a thioesterase (TE)
domain.[13]
It does not contain an enoyl reductase (ER) domain which would be
required for the complete reduction to the nine carbon fatty acid.
MupE is a single domain protein that shows sequence similarity to
known ER domains and may complete the reaction.[13]
It also remains possible that 9-hydroxy-nonanoic acid is derived
partially or entirely from outside of the mupirocin cluster.

Figure 5. MmpB is proposed to synthesize 9-HN with a
3-hydroxy-propionate starter unit and three malonyl-CoA extenter
units. The domain structure of MmpB is shown below along side with
MupE, the proposed enoyl reductase required for complete saturation
of 9-HN. ACP=acyl carrier protein, DH=dehydratase, ER=enoyl
reductase, KR=ketoreductase, KS=ketosynthase, TE=thioesterase.

Figure 5. MmpB is proposed to synthesize 9-HN with a
3-hydroxy-propionate starter unit and three malonyl-CoA extenter
units. The domain structure of MmpB is shown below along side with
MupE, the proposed enoyl reductase required for complete saturation
of 9-HN. ACP=acyl carrier protein, DH=dehydratase, ER=enoyl
reductase, KR=ketoreductase, KS=ketosynthase,
TE=thioesterase.